I have a 5 gallon tank for my one betta fish and he has no tank mates. If I ever had to treat him with AQ or Epsom salt for any reason, is it a good idea to do this in his regular 5 gallon tank or do you move them to a hospital tank reguardless of whether or not they have tank mates?

Also, I ordered some silk plants to add to my tank and also to replace the one plastic plant I have in there. How do I go about adding and moving the decorations in his tank?

It's better to move him to a smaller tank, you'll have to do 100% changes after all. Especially if you have a filter, the BB on the gravel will die if you clean it. You'll want to move him whether or not he has tank mates or none.

Just rinse the decorations in hot tap water and then take out the other decorations and replace.

Thank you!
So can I do that with my hands? Would it be okay or better to do it during a 50% water change, and should I worry too much about taking his old plant out? He does not seem to like the other tank decoration that I added. He only uses his old ones.

I use kitchen tongs to move things around in my 5 gallon but yes you can use your hands just make sure they are very clean and rinsed well with no soap on them. It may take a couple of days for him to get used to the new plants he may even flare at them but dont worry he will be fine.